Science Experiments - Lunar Surface Magnetometer

The magnetic field of the Moon was measured at four Apollo landing sites. On Apollo 12, 15, and 16, the Lunar Surface Magnetometer was included in the ALSEP experiment package. The measured magnetic field has contributions both from the Moon's intrinsic magnetic field as well as from external sources, primarily the Earth and Sun. These external sources vary with time as the Moon orbits Earth and moves through its magnetosphere. By making measurements over several months, these time-varying fields can be separated from the Moon's steady, intrinsic magnetic field. On Apollo 14 and 16, the Lunar Portable Magnetometer was carried to measure how the Moon's magnetic field varies in local regions. For these measurements, the contributions of external field sources were estimated using previously deployed ALSEP magnetometers and measurements by Explorer 35, which orbited the Moon.

The results of these experiments indicated significant variations in the strength of the Moon's magnetic field on both local and regional scales. The field strength varied from a low of 6 gammas at the Apollo 15 site to a high of 313 gammas at the Apollo 16 site. For comparison, the Earth's magnetic field strength is about 100 times higher than the highest value measured on the Moon. At the Apollo 14 site, the field strength varied between 43 and 103 gammas at two points separated by just 1.1 kilometers. At the Apollo 16 site, the field strength varied between 121 and 313 gammas at points separated by a maximum of 7.1 kilometers. These variations indicate the presence of strong localized sources in the crust for the Moon's magnetic field, a conclusion that is also consistent with observations from lunar orbit. In contrast, the Earth's magnetic field is generated by flow of fluids in the core and has a global dipole geometry. There is no evidence for such a dipole field on the Moon.

The magnetometer observations can also help constrain the interior structure of the Moon. From measurements of changes in the magnetic field at the lunar surface and in orbit as the Moon enters or exits the Earth's magnetic field, it is possible to estimate how the Moon's electrical conductivity varies with depth. This technique is known as electromagnetic sounding. The electrical conductivity varies with both chemical composition and with temperature. It has been used to place limits on the size of the Moon's iron core and on temperatures within the mantle. Mantle temperature estimates depend somewhat on the relative amount of olivine and pyroxene in the mantle. The temperature is below 1000°C until a depth of at least 500 kilometers. Temperature estimates from electromagnetic sounding approach the melting point at a depth between 800 and 1500 kilometers. For comparison, the passive seismic experiment indicated melting beginning near 1000 kilometers depth. The magnetometer observations do not require the existence of an iron-rich core, but are consistent with a core that is up to about 450 kilometers in radius. Other constraints on the size of the core come from the Passive Seismic Experiment and the Laser Ranging Retroreflector Experiment.